Can't beleive no one mentioned this one, but one thing I have found MANY times... when playing live in an air conditioned club, you are backstage in the airconditioning, tuning... everything is good. Get on stage. Hot, close lights. Pretty damn hot (i hate em) You suddenly find yourself out of tune. The heat differences can more than slightly detune you. SOmetimes its pretty bad.
Stay out of the lights, and way from them if they are pretty close. Generally I can feel them burning hles in my back pretty quickly.
The 1st time I experienced the "why" of this phenomenon... we had acoustics, camping. I noticed that the sound, as I was playing, would change key nearly half a step when I'd turn certain angles towards the heat of the fire and then away towards the somewhat chilly night. Pretty fast, actually, and it doesnt change evenly across the board.
Keep it in mind.